Modern vehicles have a plurality of electronic sensors for providing input data to their control computers. One class of sensors provides data relating to the rotational position of a rotating component, such as a shaft or wheel. These sensors, which may be referred to generically as tone wheel sensors, are positioned in proximity to a toothed or slotted metal target wheel, which is either formed integral to the rotating component or attached to it. As the target wheel rotates with the component, the tone wheel sensor emits an electrical pulse each time a tooth or slot passes it. The time periods of the electrical pulses are used by the vehicle computer to determine the rotational speed or position of the rotating component as is known in the art. Such systems are commonly used to determine the positions of the camshaft and crankshaft in the engine, the speeds of various shafts in the transmission, and the speed of each road wheel.
From time to time, it becomes necessary to diagnose a tone wheel position sensor to determine whether it is malfunctioning. Known methods of diagnosing the tone wheel sensors include removing a suspect sensor and replacing it with one that is known to be good, measuring the sensor voltages with a voltmeter, or observing sensor signals with an oscilloscope. Each of these methods has undesirable aspects. For example, replacing the tone wheel sensor requires that a technician have a known-good sensor on hand and that he expend valuable time swapping the sensors only to determine if the old one was the cause of a problem. If the old sensor was good, then the technician wasted time in replacing it.
The method of using a voltmeter also has undesirable aspects. For example, a voltmeter display typically does not have sufficient bandwidth to track each pulse from a tone wheel sensor. Even if it did, it is unlikely that a technician could comprehend a voltmeter reading that changes so rapidly. This makes the voltmeter practically useless for determining whether a sensor is intermittently malfunctioning due to vibration, etc. In addition, a voltmeter does not provide power to the sensor, nor does it provide the proper load to the sensor signal. Therefore, many types of position sensors will have to be tested while they are connected to the vehicle wiring harness through a breakout box, which provides access to the signals.
Using an oscilloscope also presents the issue of providing power to the position sensor and using a breakout box to reach the signal. Furthermore, oscilloscopes are among the most difficult to use and expensive tools in an automotive electronics tool assortment. This means that many repair facilities may either not have an oscilloscope or not have a technician who is able to use one efficiently.